Blizzard does not recognize the transfer of Accounts. You may not purchase, sell, gift or trade any Account, or offer to purchase, sell, gift or trade any Account, and any such attempt shall be null and void. Blizzard owns, has licensed, or otherwise has rights to all of the content that appears in the Program. You agree that you have no right or title in or to any such content, including the virtual goods or currency appearing or originating in the Game, or any other attributes associated with the Account or stored on the Service. Blizzard does not recognize any virtual property transfers executed outside of the Game or the purported sale, gift or trade in the “real world” of anything related to the Game. Accordingly, you may not sell items for “real” money or otherwise exchange items for value outside of the Game.

Also, the recent eBay delisting of all virtual property has really hurt the big gold sellers, such as IGN.

Selling virtual goods, while benefiting sellers, does hurt the in-game economy for those that play the game. This is the main reason that Blizzard is against the sale of virtual property.